The proposed studies investigate the interaction of Toxoplasma gondii and commensal bacterial flora in the immune regulation of an experimental model of pathogen-driven inflammatory bowel disease (IBD). The hypothesis to be tested is that specific bacterial products derived from commensal intestinal flora can interact with the parasite-infected host and prevent the development of experimental IBD. We have observed that germ free mice are more susceptible to an acute necrotizing condition of the ileum and colon than conventional mice following oral parasite infection suggesting that intestinal microflora or their derived products are required to prevent the development of Toxoplasma induced IBD. In the first specific aim, we will compare intestinal tissue samples (phenotyping, chemokines cytokines production) in germ-free strains of resistant (BALB/c) and susceptible (C57BL/6) mice following oral Toxoplasma infection to conventional mice. We have identified a specific capsular polysaccharide of B. fragilis that can modulate the gut inflammatory process. Treatment of conventional mice with capsular polysaccharide (PS A) derived from Bacteroides fragilis prevents the development of T. gondii induced IBD. The mucosal tissue and more specifically lymphoid cells from the lamina propria, Peyer's patches and other organs from PS A treated conventional and germ free mice will be assessed for immunohistologic differences. Mechanisms (IL-10, TGF-b production) of immunomodulation induced by the PS A will be further investigated. As suggested by our preliminary data, particular attention will focus on TGF-b producing CD8+ intraepithelial lymphocytes (IEL) and on a population of IL-10 dependent CD4 regulatory T cells (CD45RB low, CD25). We will evaluate for the expression of these regulatory CD4+ T cells in the lamina propria and Peyer's patches of PS A treated mice and determine whether this cell population can be adaptively transferred to naive mice and prevent toxoplasma-driven IBD. NKT cells are also important regulatory cells that respond to polysaccharides. We will evaluate whether PS A treatment can induce a population of NKT cells that exert effector and immunoregulatory activity.